1. Field of the Invention
The present invention relates to a method for driving an LCD monitor, and more particularly, to a method for obtaining image quality of specified driving methods (such as a line inversion driving method) with power consumption of a frame inversion driving method.
2. Description of the Prior Art
The advantages of a liquid crystal display (LCD) include lighter weight, less electrical consumption, and less radiation contamination. Thus, the LCD monitors have been widely applied to various portable information products, such as notebooks, PDAs, etc. In an LCD monitor, incident light produces different polarization or refraction effects when the alignment of liquid crystal molecules is altered. The transmission of the incident light is affected by the liquid crystal molecules, and thus magnitude of the light emitting out of liquid crystal molecules varies. The LCD monitor utilizes the characteristics of the liquid crystal molecules to control the corresponding light transmittance and produces gorgeous images according to different magnitudes of red, blue, and green light.
Please refer to FIG. 1, which illustrates a schematic diagram of a prior art thin film transistor (TFT) LCD monitor 10. The LCD monitor 10 includes an LCD panel 100, a control circuit 102, a data-line-signal output circuit 104, a scan-line-signal output circuit 106, and a voltage generator 108. The LCD panel 100 is constructed by two parallel substrates, and the liquid crystal molecules are filled up between these two substrates. A plurality of data lines 110, a plurality of scan lines 112 that are perpendicular to the data lines 110, and a plurality of TFTs 114 are positioned on one of the substrates. There is a common electrode installed on another substrate, and the voltage generator 108 is electrically connected to the common electrode for outputting a common voltage Vcom via the common electrode. Please note that only four TFTs 114 are shown in FIG. 1 for clarity. Actually, the LCD panel 100 has one TFT 114 installed in each intersection of the data lines 110 and scan lines 112. In other words, the TFTs 114 are arranged in a matrix format on the LCD panel 100. The data lines 110 correspond to different columns, and the scan lines 112 correspond to different rows. The LCD monitor 10 uses a specific column and a specific row to locate the associated TFT 114 that corresponds to a pixel. In addition, the two parallel substrates of the LCD panel 100 filled up with liquid crystal molecules can be considered as an equivalent capacitor 116.
The operation of the prior art LCD monitor 10 is described as follows. When the control circuit 102 receives a horizontal synchronization signal 118 and a vertical synchronization signal 120, the control circuit 102 generates corresponding control signals respectively inputted into the data-line-signal output circuit 104 and the scan-line-signal output circuit 106. The data-line-signal output circuit 104 and the scan-line-signal output circuit 106 then generate input signals to the LCD panel 100 for turning on the corresponding TFTs 114 and changing the alignment of liquid crystal molecules and light transmittance, so that a voltage difference can be kept by the equivalent capacitors 116 and image data 122 can be displayed in the LCD panel 100. For example, the scan-line-signal output circuit 106 outputs a pulse to the scan line 112 for turning on the TFT 114. Therefore, the voltage of the input signal generated by the data-line-signal output circuit 104 is inputted into the equivalent capacitor 116 through the data line 110 and the TFT 114. The voltage difference kept by the equivalent capacitor 116 can then adjust a corresponding gray level of the related pixel through affecting the related alignment of liquid crystal molecules positioned between the two parallel substrates. In addition, the data-line-signal output circuit 104 generates the input signals, and magnitude of each input signal inputted to the data line 110 is corresponding to different gray levels.
If the LCD monitor 10 continuously uses a positive voltage to drive the liquid crystal molecules, the liquid crystal molecules will not quickly change a corresponding alignment according to the applied voltages as before. Thus, the incident light will not produce accurate polarization or refraction, and the quality of images displayed on the LCD monitor 10 deteriorates. Similarly, if the LCD monitor 10 continuously uses a negative voltage to drive the liquid crystal molecules, the liquid crystal molecules will not quickly change a corresponding alignment according to the applied voltages as before. Thus, the incident light will not produce accurate polarization or refraction, and the quality of images displayed on the LCD monitor 10 deteriorates. In order to protect the liquid crystal molecules from being irregular, the LCD monitor 10 must alternately use positive and the negative voltages to drive the liquid crystal molecules. In addition, not only does the LCD panel 100 have the equivalent capacitors 116, but the related circuit will also have some parasite capacitors owing to its intrinsic structure. When the same image is displayed on the LCD panel 100 for a long time, the parasite capacitors will be charged to generate a residual image effect. The residual image with regard to the parasite capacitors will further distort the following images displayed on the same LCD panel 100. Therefore, the LCD monitor 10 must alternately use the positive and the negative voltage to drive the liquid crystal molecules for eliminating the undesired residual image effect. Please refer to FIG. 2 and FIG. 3, FIG. 2 and FIG. 3 are schematic diagrams of a prior art frame inversion driving method. Blocks 20 and 30 show polarities of pixels in the same part of two successive image frames. Comparing the blocks 20 and 30, when the LCD panel 100 is driven by the frame inversion driving method, polarities of pixels in a frame are uniform and change to opposite polarities as a frame changes.
However, when the LCD monitor 10 alternately uses the positive and negative voltage to drive the liquid crystal molecules, the image displayed will flicker owing to a voltage offset generated by the TFT 114. The reason is described as follows. Firstly, as shown in FIG. 1, the gray level variation of each pixel is generated by the equivalent capacitor 116 with different voltages, which is driven by the corresponding TFT 114. Practically, the TFT 114 is also affected by spurious elements, such as off resistances (Roff) and gate-drain capacitors (Cgd), so that the voltages outputted to the equivalent capacitor 116 are offset. For example, please refer to FIG. 4, which is an output voltage diagram of the data-line-signal output circuit 104 shown in FIG. 1. As with the voltages V0, V1, V2, V3, V4, V5, V6, V7, V8, V9 shown in FIG. 4, the data-line-signal output circuit 104 generates different voltages according to display data 122 for driving the TFTs 114 positioned on the LCD panel 100. However, when the thin film transistor 114 is turned on, the voltage difference between the input terminal and the output terminal of the TFT 114 is equal to a deviation Vd. Therefore, the actual values of voltages such as V20, V21, V22, V23, V24, V25, V26, V27, V28, V29 imposed on the LCD panel 100 are individually lower than the corresponding ideal values of voltages such as V0, V1, V2, V3, V4, V5, V6, V7, V8, V9. As mentioned above, the LCD monitor 10 alternatively uses the positive and negative voltages to drive each pixel on the LCD panel 100. In other words, the voltage outputted from the data-line-signal output circuit 104 has to be changed so that the voltage difference between the voltage outputted from the data-line-signal output circuit 104 and the common voltage Vcom generated by the voltage generator 108 has an alternating polarity. For example, the display data 122 indicates that a voltage difference V1−Vcom is required to drive one pixel, and the pixel will hold the voltage difference V1−Vcom during a predetermined interval. Because the pixel is alternatively driven with the positive and negative voltages, the positive voltage V1−Vcom and the negative voltage −(Vcom−V8) are alternatively imposed on the LCD panel 100. However, the actual voltage V21−Vcom is not equal to the voltage Vcom−V28 owing to the deviation Vd of the TFT 114. Therefore, when the pixel is alternatively driven with the positive voltage V21−Vcom and the negative voltage −(Vcom−V28), the pixel flickers because of an unstable gray level.
In order to solve the mentioned problem when the LCD monitor 10 alternatively uses the positive and negative voltages to drive the liquid crystal molecules, the LCD monitor 10 adopts different driving methods to eliminate the image flickers. Please refer to FIG. 5 to FIG. 6. FIG. 5 and FIG. 6 are diagrams of a prior art line inversion driving method. Blocks 50 and 60 show polarities of pixels in the same part of two successive image frames. Comparing the blocks 50 and 60, when the LCD panel 100 is driven by the line inversion driving method, polarities of pixels in a line are uniform and change to opposite polarities as a frame changes. Nevertheless, polarities of pixels in adjacent lines are opposite.
As the LCD panel is driven by the line inversion driving method, polarities of pixels in a line are uniform and change to opposite polarities as a frame changes, and polarities of pixels in adjacent lines are opposite. Hence, the line inversion driving method can eliminate image flickers along the vertical direction. Therefore, the line inversion driving method achieves better image quality than the frame inversion driving method. However, the line inversion driving method consumes more power than the frame inversion driving method does, so that applications of the line inversion driving method are limited, especially in portable electric devices.